bpf: Clarify get_outer_instance() handling in propagate_to_outer_instance()

propagate_to_outer_instance() calls get_outer_instance() and uses the
returned pointer to reset and commit stack write marks. Under normal
conditions, update_instance() guarantees that an outer instance exists,
so get_outer_instance() cannot return an ERR_PTR.

However, explicitly checking for IS_ERR(outer_instance) makes this code
more robust and self-documenting. It reduces cognitive load when reading
the control flow and silences potential false-positive reports from
static analysis or automated tooling.

No functional change intended.

Signed-off-by: Shardul Bankar <[email protected]>
Acked-by: Eduard Zingerman <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

Author: shardulsb08

kernel/bpf/liveness.c

@@ -524,6 +524,8 @@ static int propagate_to_outer_instance(struct bpf_verifier_env *env,
 
 	this_subprog_start = callchain_subprog_start(callchain);
 	outer_instance = get_outer_instance(env, instance);
+	if (IS_ERR(outer_instance))
+		return PTR_ERR(outer_instance);
 	callsite = callchain->callsites[callchain->curframe - 1];
 
 	reset_stack_write_marks(env, outer_instance, callsite);